Proton conducting membranes (PEM) are widely utilized in electrochemical devices which employ a chemical reaction to produce or store electricity. Exemplary electrochemical devices include fuel cells, electrolysis cells, hydrogen separation cells, and batteries.
An increasingly important use for PEM materials is in fuel cells. A fuel cell generates electricity from the electrochemical reaction of a fuel (e.g., hydrogen, methane or methanol) and oxygen. A fuel cell contains a PEM interposed between an anode and a cathode, each contained in its own compartment. The anode and the cathode are connected through an external circuit which can have a load such as an electric drive motor. Anodes and cathodes are generally coated with precious metals such as platinum to catalyze the electrochemical reactions occurring at the anode and cathode. At the anode, hydrogen (from the fuel source) is oxidized to protons and electrons. The electrons are conducted by the anode through the external load and back to the cathode. The protons are transported directly across the PEM to the cathode where they are combined with electrons (returning from the external load) and oxygen to form water. The ability of the PEM to effectively conduct protons to the cathode while acting as an impermeable barrier to fuel cell gases and liquids are integral factors in maintaining fuel cell efficiency. The flow of current is sustained by a flow of protons across the PEM and electrons through the external load. Theoretically, fuel cells can produce power continuously so long as the supply of fuel and oxygen is sustained and the PEM material maintains its physical integrity and proton conducting efficiency. All fuel cells are limited by the performance of the PEM.
There are many types of fuel cell configurations in common use (e.g., direct hydrogen/air fuel cell, indirect hydrogen/air fuel cell, and organic fuel cell), each having associated advantages and disadvantages. One type of fuel cell is the direct methanol fuel cell (DMFC). A DMFC utilizes methanol as the proton source. An aqueous solution of methanol is directly fed into cell, where the fuel is oxidized at the anode to produce CO2, electrons and protons. The protons are transported across the PEM where oxygen is reduced to water at the cathode.
The PEM plays a very important role in the operation of fuel cells. On one hand it acts as a proton conducting medium, permitting the transfer of hydrated protons (H3O+) from the anode to the cathode, and on the other hand it functions as a barrier that is impermeable to fuel cell gases and liquids. The PEM must meet many specifications relating to mechanical, chemical, and electrical properties. For example, the polymer must be able to be cast into thin films without defects. The mechanical properties must permit the membrane to withstand assembly operations such as being clamped between metal frames. The polymer must have good stability to hydrolysis and exhibit good resistance to harsh chemical reactions such as oxidation and reduction. The polymer must exhibit good thermal stability as well as a need to endure wide fluctuations in temperature conditions. The PEM must also have an affinity for hydration since the transport of protons across the polymer membrane occurs in the form of hydronium ions in aqueous medium. Finally, the PEM must have high proton conductivity or the ability for proton transport across the membrane. This conductivity is provided by the ability to functionalize the polymer with strong acidic groups.
Heretofore, various polymers have been utilized for the PEM but with only limited success. One such polymer is Nafion® (available from DuPont) which is a sulfonated poly(perfluoroethylene). Despite this limited success, Nafion polymers are generally considered to be the current standard PEM. However, the use of such perfluoroethylene polymers as PEM's can be problematic.
For example, while in many current applications the membrane is maintained at an operating temperature close to ambient (i.e., not exceeding 80° C.), higher operating temperatures (approaching 120° C. and above) are desirable from the standpoint of increasing catalyst efficiency at the anode. Perfluoroethylene polymers such as Nafion generally suffer from poor thermal stability and mechanical strength at such higher operating temperatures. Generally, after thirty days of continuous exposure to operating temperatures of 120° C. perfluoroethylene polymers are virtually unusable. It is believed that such poor thermal stability and mechanical strength of perfluoroethylene polymers are due to their lack of a crosslinked structure.
Another issue with membranes fabricated from perfluoroethylene polymers arises from the requirement to maintain high levels of moisture within the membrane. A high level of hydration is necessary to facilitate trans-membrane proton transport, while reduced levels of hydration results in decreased proton transport efficiency. Accordingly, it is necessary to humidify the membrane during fuel cell operation to maintain transport efficiency. This requires additional equipment to regulate and maintain the overall water balance requirements of the fuel cell. However, as temperatures in the fuel cell are increased to take advantage of higher catalyst efficiencies, an attendant decrease in humidity levels occurs within the cell. Consequently, the fuel cell must be pressurized when cell temperatures exceed 100° C.
Another problem found with perfluoroethylene polymers is in their use in direct methanol fuel cells. Since perfluoroethylene polymers can be permeable to methanol, methanol can leak from the anode compartment across the membrane into the cathode compartment reducing fuel cell efficiency.
Recently, on Jan. 23, 2001, a new PEM material was disclosed in Japanese published Patent Application No. 2001-019723, assigned to Toyota Central Research & Development Lab Inc. The PEM of this application is a copolymer formed of norbornene monomers with an olefinic monomer such as trifluorostyrene. As with Nafion perfluoroethylene polymers, the polymer disclosed in the Toyota application is not crosslinked. In addition, the disclosed polymer contains only one type of functionality pendant from the polymer backbone (i.e., a sulfonic acid functionality added to the phenyl ring of the styrenic repeating unit). Therefore it would appear that this polymer would suffer some of the same drawbacks of the Nafion polymers.
Accordingly, there is still an unsatisfied need for new polymers which can be readily fabricated into thin film membranes and which can be tailored to meet the stringent conditions required by operating fuel cells. Such thin film membranes should require little or no additional humidification, and should be capable of being operated at elevated temperatures, for example in excess of 120° C., and/or they should be more resistant to methanol permeability than Nafion type polymer membranes, advantageously making them advantageous for proton conducting membranes of fuel cells and the like.